There are a number of recognized ways to convert digital data to an analog signal. Included are the well-known R-2R networks using a binary scale of voltages and currents. Binary weighted currents flowing in a given position may be routed to ground or to the summing input of an operational amplifier. The positions of switches reproduce the code of the digital information to be converted. A series of MOS transistors may be used as the switches, but as currents increase, the design becomes complicated in order to keep voltage drops constant. In addition, a series of resistors may be needed to keep the emitters (or sources) of the series of transistors at the same potential. The accuracy of R-2R circuits is based on the tracking of the resistances. It may be difficult to achieve 10 or 12 bit conversion.
Switched capacitors are also used to perform digital-to-analog conversion. Capacitors may be integrated and have less dependence on temperature than resistive circuits, especially thin-film resistors. Capacitors suffer other drawbacks, however, and require special care in order to provide reasonable speed and good tracking. Integrated capacitors are sandwiches of metal-oxide-silicon (MOS technology) or polysilicon-oxide-polysilicon (double poly technology). These latter exhibit a highly linear behavior and extremely small temperature coefficients of performance, typically 10 to 20 ppm per degree Celsius. In order to improve geometrical tracking, capacitors must represent a combination of xe2x80x9cunitxe2x80x9d capacitance, where a unit capacitance represents the minimum-sized element available, typically 100 fF. MOS capacitors have relatively large stray capacitances to the substrate through their inversion layer, typically 10 to 30% of their nominal capacitance value, depending on the oxide thickness. Double poly capacitors offer better performance, but the manufacturing process is far more complicated and expensive, with greater chances for errors. Digital-to-analog converters using capacitor integration must also be designed to tolerate stray capacitance.
Another widely used technique to transform digital data to an analog signal is to parallel many identical transistors, controlled by a single base-to-emitter or gate-to-source voltage source. The output terminals are tied together to implement banks of binary weighted current sources. This requires 2N transistors to make an N-bit converter. For example, an 8-bit converter requires a series of 28, or 256 transistors. However, in MOS technology, transistors are the smallest on-chip devices available, and the technique is an accepted one. In addition, MOS manufacturing techniques tend to be robust, and it is far easier to attain 10-bit accuracy, for example, than with capacitance MOS or double poly techniques. Also this technique is preferred for high speed digital to analog conversion since it uses an open loop structure i.e no feedback is used which might limit the operation speed significantly.
Some of the disadvantages of these techniques may include the need for trimming of resistors to insure that each transistor is kept at the same potential. Trimming requires more processing and even laser trimming in some cases. Better performance is achieved by keeping the gate-to-source potential for each transistor the same, that is, keeping the gate-to-source potential constant. What is needed is a better way to keep the gate-to-source potential of each transistor the same and thus to better perform digital to analog conversion.
The present invention meets this need by providing a current source for the gates in a series of transistors in a digital-to-analog converter. In one embodiment, a current source is provided for a digital-to-analog converter cell row. The circuit includes a power source, a series of MOS transistors, and a path to ground. The source of a first power transistor in the series is connected to its own gate and to the gate of the next transistor. Thereafter, the gate of each transistor is connected to the gate of the next transistor in the series. All sources of the transistors are connected are connected to the power source. The gate-to-source voltage of each transistor in the circuit is the same, constant, because the resistor in each circuit has the same value, and the current flowing in the circuit is the same. Thus, the constant drop from transistor to transistor is the same, and each transistor has a constant gate-to-source value.
In another embodiment, the resistors are eliminated, and the gate of each transistor is connected directly to a constant potential. The sources in a series of transistors are connected to a busbar at a potential, and the busbar is tapered. The busbar supplies both voltage and current, frequently 10-20 mA or more, and is tapered from wide at a near end near the source voltage source, to narrow at an end away from the source voltage. Therefore, the current flow in the first transistors at the near end does not affect the voltage available to the transistors at the far end. Without this taper, the current draw from the first transistors may cause an IR drop through the busbar, resulting in a lower potential at the far end, and thus a lower source voltage available to the transistors.
In another embodiment, the busbar for the sources is not tapered, but the busbar for the gates has an inverse taper, that is, it is narrowest at an end near the gate voltage source and has an increasing taper at a far end, away from the gate voltage source. In this manner, the potential at each source is slightly less than the previous source; and the potential at each gate is slightly less than the previous gate. Thus, the source-to-gate voltage is constant. When the digital-to-analog (DAC) switches are activated, the source-to-gate voltages are also constant; that is, they are the same from transistor to transistor in the DAC.